The present invention relates generally to the field of variable geometry turbocharger design and, more particularly, to method and system for controlling the position of a variable geometry member disposed within a variable geometry turbocharger.
Turbochargers are devices that are frequently used to increase the output of an internal combustion engine. A typical turbocharger comprises a turbine wheel coupled to a compressor impeller by a common shaft. Exhaust gas from the engine is diverted into a turbine housing of the turbocharger and through an inlet nozzle. The exhaust gas is directed onto the turbine wheel, causing it to spin, which in turn spins the common shaft and the compressor impeller.
The compressor impeller is disposed within a compressor housing having an air inlet and a pressurized or boosted air outlet. The spinning compressor impeller operates to pressurize air entering the compressor housing and generate a pressurized or boosted air stream that is directed into an inlet system of the internal combustion engine. This boosted air is mixed with fuel to provide a combustible mixture within the combustion chambers of an engine. In this manner, the turbocharger operates to provide a larger air mass and fuel mixture, than otherwise provided via an ambient pressure air intake stream, that results in a greater engine output during combustion.
The gain in engine output that can be achieved is directly proportional to the increase in intake air flow pressure generated by the turbocharger. However, allowing the boost pressure to reach too high a level can result in severe damage to both the turbocharger and the engine, particularly when the engine has to operate beyond its intended performance range.
Thus, an objective of turbocharger design is to regulate or control the boost pressure provided by the turbocharger in a manner that optimizes engine power output at different engine operating conditions without causing engine damage. A known technique for regulating boost pressure is by using a turbocharger having a variable geometry member that functions to control the amount of exhaust gas directed to the turbine wheel. Turbochargers comprising such variable geometry members are referred to as variable geometry turbochargers (VGTs).
One type of VGT includes a variable geometry member in the form of multiple adjustable-position vanes that are positioned within the turbine housing, and that are movable within an inlet nozzle of the turbine housing to regulate the amount of exhaust gas that is passed to the turbine wheel. The vanes in this type of VGT can be opened to permit greater gas flow across the turbine wheel, causing the turbine wheel to spin at a higher speed and raise the boost pressure, or closed to restrict exhaust gas flow to the turbine, thereby reducing the boost pressure. Thus, the amount of boost pressure generated by this type of VGT can be regulated by varying the vane position so as to optimize engine output while avoiding engine damage.
However, it is important for purposes of obtaining the desired result from the VGT and the desired output from the engine that the variable geometry member, in this or any type of VGT, be operated in a manner that will produce the desired change. Since this operation is taking place in a dynamic system of changing VGT and engine operating parameters, it is desired that a control system be used for the purpose of taking these dynamic operating conditions into account so as to provide the desired result.
It is, therefore, desired that a control system be devised that is capable of being used with a VGT to effect a desired change in the variable geometry member disposed therein for the purpose of achieving a desired VGT and engine output.
The present invention discloses methods and systems for variable geometry turbocharger (VGT) control. In one embodiment of the invention, a boost target for the turbocharger is determined from a boost target map, for example. The boost target is then compared to the actual boost to calculate an error value, errboost, between the boost target and the actual boost. Based on errboost, a first new VGT variable geometry member, e.g., vane, position is determined by way of a conventional proportional integral differential (PID) technique, for example.
Alternatively, the first new variable geometry member position can be generated using a modified PID technique, whereby a change in variable geometry member position is calculated according to the equation, xcex94xcex8=kp(errboost)+kdxc2x7d(errboost)/dt, where xcex94xcex8 is the change in variable geometry member position, kp is a proportional gain value, kd is a differential gain value, and errboost is the error value between the boost target and the actual boost. Following, xcex94xcex8 is summed with the preceding variable geometry member position to determine the first new variable geometry member position. The first new variable geometry member position may be modified by a feed forward value, FF, set as a function of the absolute value of change in fuel rate or throttle position, a threshold value, and a constant. A second new variable geometry member position may also be generated as a function of the engine speed of the engine. The variable geometry member of the VGT is then positioned by an actuator to the first new variable geometry member position if the engine is in a power mode, and to the second new variable geometry member position if the engine is in a braking mode.
In another embodiment of the invention, a boost target for the VGT is determined from a boost target map. The boost target is then compared to the actual boost to calculate a first error value, errboost, between the boost target and the actual boost. Based on errboost, a turbo speed target is determined by way of a conventional PID technique, for example. A second error value, errboost, is then calculated between the turbo speed target and the actual turbo speed of the turbocharger. The errboost may then be inputted into a PID module to determine a new variable geometry member position for the turbocharger using a conventional PID technique. In certain embodiments, the turbo speed target and the new variable geometry member position may be generated using a modified PID technique. For example, the turbo speed target can be generated by first determining a change in turbo speed, xcex94speed, where xcex94speed is substantially equal to kp(errboost)+kdxc2x7d(errboost)/dt, and then summing xcex94speed with the actual turbo speed. The new variable geometry member position, meantime, may be generated by first calculating a change in variable geometry member position, xcex94xcex8, where xcex94xcex8 is substantially equal to kp(errspeed)+kdxc2x7d(errboost)/dt, and then summing xcex94xcex8 with a preceding variable geometry member position. An actuator may then be used to position the variable geometry member of the turbocharger according to the new variable geometry member position.
In yet another embodiment of the invention, a boost target for the VGT is determined, and an error value, errboost, between the boost target and the actual boost is calculated. A first turbo speed target, based on errboost, is then generated using, for example, a conventional PID technique. Additionally, a turbine pressure target for the turbocharger is determined from a turbine pressure map. The turbine pressure target is then compared to the actual turbine pressure in order calculate a second error value, errturbine, which is then used to generate a second turbo speed target. As an example, the second turbo speed target can be generated using a conventional PID technique. Following, if the engine is in power mode, then the first turbo speed target is selected for use is determining a new variable geometry member position. However, if the engine is in braking mode, then the second turbo speed target is the selected turbo speed target. Depending on the mode of the engine, the selected turbo speed target is then compared to the actual turbo speed to calculate a third error value, errspeed, which is used to determine a new variable geometry member position for the turbocharger.
In certain aspects of the present embodiment, rather than using conventional PID techniques, the first and second turbo speed target and the new variable geometry member position can be generated using a modified PID technique. For instance, the change in turbo speed needed to achieve the first turbo speed target may first be determined and then summed with the actual turbo speed to generate the first turbo speed target, wherein the change in turbo speed, xcex94speed, is substantially equal to kp(errboost)+kdxc2x7d(errboost)/dt. In a parallel manner, the second turbo speed target can be generated by adding the change in turbo speed, xcex94speed, to the actual turbo speed, where xcex94speed is substantially equal to kp(errturbine)+kdxc2x7d(errturbine)/dt. The new variable geometry member position, meantime, may be generated by first calculating the change in variable geometry member position, xcex94xcex8, and then summing xcex94xcex8 with the variable geometry member position of the preceding iteration, where xcex94xcex8 is substantially equal to kp(errspeed)+kdxc2x7d(errspeed)/dt.